Polymer adhesion modulus

It is interesting to compare the dependence of the strength of interfacial layer on its structural characteristics for the polymer-polymer and polymer-high-modulus filler. For the first type of systems with increasing Ij there is observed the growth of adhesion [17]. 

In effect, this theory postulates a chemical reaction between promoter, polymer, and mineral substrate as in the chemical bond theory but also suggests that the presence of a region of intermediate modulus between polymer and substrate which transfers stress from the high modulus surface to the relatively low modulus polymer. Adhesive technology has long recognized this principle in specially formulated primers for use when bonding rubbery polymers to metals. 

Figure 4.2 Dependence of the polymer elastic modulus in the adhesive layer on the distance to the substrate. The modulus is determined by the dynamic (1) and static (2) methods.

Couphng agents are used to modify polymers to provide sites for reaction (see Section 3.4) or to promote adhesion (see Section 12.7). They are more frequently used to modify filler surfaces to improve incorporation and distribution, to improve electrical properties, and, in some cases, to cause bonding of the filler to the polymer, improving modulus and resistance to deformation. A coupling agent has the general formula... 

We have recently been exploring this technique to evaluate the adhesive and mechanical properties of compliant polymers in the form of a nanoscale JKR test. The force and stiffness data from a force-displacement curve can be plotted simultaneously (Fig. 13). For these contacts, the stiffness response appears to follow the true contact stiffness, and the curve was fit (see [70]) to a JKR model. Both the surface energy and modulus can be determined from the curve. Using JKR analyses, the maximum pull off force, surface energy and tip radius are... 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

While there are a large number of elastomers that can be formulated into pressure sensitive adhesives, the following list is intended to focus on commercially significant materials. Two subsets are differentiated in Table 1 those polymers that can be inherently tacky, and those that require modification with tackifiers to meet the Tg and modulus criteria to become pressure sensitive. 

Lowering of the rubbery plateau modulus increases the compliance of the polymer making faster wet-out of a substrate possible. As a result, the PSAs show more aggressive tack properties. Provided the surface energy of the substrate allows for complete polymer wetting, a PSA with improved quick-stick and faster adhesion build will be obtained. 

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